The present invention relates to a phosphor precursor. Specifically, the invention relates to spherical particles of a precursor of a rare earth activated barium fluoride halide phosphor.
It is known that a rare earth activated barium fluoride halide phosphor gives spontaneous emission in the ultraviolet or visible wavelength region when exposed to radiation such as X-rays and electron beams. This phosphor is, for example, used for a radiographic intensifying screen, in which the intensifying screen is used in combination with a radiographic photosensitive material.
The above-identified phosphor is also known as a stimulable phosphor. When the stimulable phosphor is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The stimulable phosphor then emits stimulated emission to release the stored energy when exposed to electromagnetic wave such as visible light or infrared rays (i.e., stimulating light). The aforementioned phosphor is, therefore, used for preparing a radiation image storage panel (i.e., imaging plate) in a radiation image recording and reproducing method.
The radiation image recording and reproducing method has been widely employed in practice, and it comprises the steps of: causing the stimulable phosphor contained in the radiation image storage panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with a stimulating light such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric signals giving a reproduced radiation image. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
In the radiographic intensifying screen or radiation image storage panel, the rare earth activated barium fluoride halide phosphor is normally dispersed in a binder. The phosphor particles preferably are spherical so that they can be densely dispersed in the binder. If the particles have tabular shapes, they are apt to align horizontally (i.e., parallel to the plane of the screen or panel), and accordingly the emission is liable to diffuse horizontally to lower the quality (sharpness and graininess) of the resultant radiographic image. Further, the phosphor particles also preferably have such a small size-distribution (i.e., they preferably are monodispersed) that they can be evenly dispersed in a binder solution. In order to prepare the spherical and monodispersed phosphor particles efficiently and easily, it is desired that a phosphor precursor be in the form of spherical and monodispersed particles. Here, the term xe2x80x9cphosphor precursorxe2x80x9d or xe2x80x9cprecursor of phosphorxe2x80x9d means a compound that is yet to be fired, that has almost the same formula as the phosphor to be obtained by firing, but that hardly or insufficiently emits luminescence, that is xe2x80x9cnon-luminescentxe2x80x9d.
Ordinarily, for preparing the rare earth activated barium fluoride halide phosphor, starting materials such as barium fluoride, barium halide, rare earth halide and ammonium fluoride are mixed under dry or wet conditions, and are then fired. The obtained phosphor is generally in the form of tabular particles.
Japanese Patent Provisional Publication No. 7-233369 discloses a stimulable rare earth activated alkaline earth metal fluoride halide phosphor [in the formula of (Ba,MII)FX:MI,Ln in which MII is Sr and/or Ca, MI is an alkali metal, X is a halogen other than F, and Ln is a rare earth element] in the form of tetradecahedral particles and a process for a preparation thereof. The phosphor is prepared by precipitating and firing a precursor of the phosphor in the form of tetradecahedral crystals. According to the publication, the tetradecahedral crystals of the precursor can be obtained by the steps of: preparing an aqueous solution in which BaX2, a halide of Ln and, if needed, halides of MII and MI are dissolved so that the concentration of BaX2 is 1.4 mol/L or less, and adding an aqueous solution of an inorganic fluoride to the prepared halide solution, while the temperature kept at 20 to 100xc2x0 C.
Japanese Patent Provisional Publication No. 6-9956 discloses a process for preparation of a spherical rare earth activated barium fluoride halide phosphor. The process comprises the steps of: adding a mixture of phosphor particles and a binder to an organic solvent to prepare a slurry, spray-drying the slurry to form spherical binder-containing agglomerates, heating the agglomerates at a temperature of 200xc2x0 C. to 500xc2x0 C. for 0.5 to 5 hours to remove the binder, and firing the heated agglomerates at a temperature of 600xc2x0 C. to 1,300xc2x0 C. for 0.5 to 5 hours.
Japanese Patent Provisional Publication No. 10-251635 discloses a process for preparation of spherical phosphor particles having a mean size of 0.05 to 1 xcexcm. In the process, a powdery phosphor (stating material) is treated in a heated non-oxidative stream.
In all the known processes, the starting material for the preparation of a phosphor is a phosphor, and there is no description of a phosphor precursor, which is non-luminescent. Further, it has not yet been reported that a spherical phosphor precursor s produced according to the liquid phase reaction process, which needs not a large-scale apparatus used in a thermal decomposition-spraying process. Naturally, the size distribution of the resultant phosphor particles greatly depends on that of the starting materials, and therefore it is necessary that the material particles be evenly dispersed before the treatment. However, there is known no method for evenly dispersing the starting material particles, and hence the resultant phosphor particles are considerably multidispersed.
It is an object of the present invention to provide a spherical precursor of a rare earth activated barium fluoride halide phosphor particle, from which the target spherical phosphor is advantageously prepared.
It is another object of the invention to provide a process by which the spherical precursor of rare earth activated barium fluoride halide phosphor particles is easily produced.
The applicant has studied the process for preparation of a rare earth activated barium fluoride halide phosphor in the form of spherical particles. As a result, it is found that, if a water-soluble polymer material is used under properly controlled reaction conditions, the precursor of the phosphor in the form of almost spherical and monodispersed particles can be easily prepared by the liquid phase reaction process. From the obtained spherical precursor, the phosphor in the form of almost spherical and monodispersed particles can be prepared.
The invention resides in non-luminescent spherical rare earth activated barium fluoride halide particles, that is, precursor particles having the formula (I):
xe2x80x83Ba1-aMIIaFX:yMI,zLnxe2x80x83xe2x80x83(I)
in which MII is at least one alkaline earth metal selected from the group consisting of Ca and Sr; MI is at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; X is at least one halogen selected from the group consisting of Cl, Br and I; Ln is at least one rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; and a, y and z are numbers satisfying the conditions of 0xe2x89xa6axe2x89xa60.5, 0xe2x89xa6yxe2x89xa60.05, and 0 less than zxe2x89xa60.2, respectively.
In the present invention, the term of xe2x80x9cnon-luminescent spherical rare earth activated barium fluoride halide particlesxe2x80x9d means spherical rare earth activated barium fluoride halide particles which emit no luminescent radiation or emit only such a small amount of luminescence radiation that the particles per se are not practically employable for manufacturing a radiation intensifying screen.
The invention also resides in a process for preparing spherical rare earth activated barium fluoride halide phosphor particles, having the following formula (I):
Ba1-aMIIaFX:yMI,zLnxe2x80x83xe2x80x83(I)
in which MII is at least one alkaline earth metal selected from the group consisting of Ca and Sr; MI is at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; X is at least one halogen selected from the group consisting of Cl, Br and I; Ln is at least one rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; and a, y and z are numbers satisfying the conditions of 0xe2x89xa6axe2x89xa60.5, 0xe2x89xa6yxe2x89xa60.05, and 0 less than zxe2x89xa60.2, respectively; which comprises the steps of:
(1) dissolving at least a water-soluble polymer material and a halide containing an X ion in an aqueous medium, whereby preparing an aqueous solution in which the X ion is dissolved in an amount of 3.0 mol/L or more and which has a pH value in the range of 5.0 to 14.0 and a temperature in the range of 0xc2x0 C. to 80xc2x0 C.;
(2) mixing an aqueous barium compound solution, an aqueous fluoride solution, and an aqueous X ion-containing halide solution with the aqueous solution prepared in the first step while a resulting mixture solution is kept at a temperature in the range of 0xc2x0 C. to 80xc2x0 C., whereby forming barium fluoride halide particles in the mixture solution,
(3) removing the water-soluble polymer material from the mixture solution containing the barium fluoride halide particles;
(4) mixing the mixture solution from the water-soluble polymer material is removed with an aqueous solution containing a rare earth compound, an alkaline earth metal compound and an alkali metal compound, the alkaline earth metal compound being not contained in the case of a=0 and the alkali metal compound being not contained in the case of y=0, whereby precipitating spherical rare earth activated barium fluoride halide particles;
(5) separating the spherical rare earth activated barium fluoride halide particles from the resulting mixture solution; and
(6) firing the spherical rare earth activated barium fluoride halide particles whereby obtained the spherical rare earth activated barium fluoride halide phosphor particles.
The invention further resides in spherical rare earth activated barium fluoride halide phosphor particles, prepared by firing the above precursor and having the above-mentioned formula (I).
It should be noted that the component ratio of the precursor particle may be slightly different from that of the resultant phosphor particle because the chemical composition may vary during the firing procedure for producing the phosphor.